Devices for moving cams relative to their driving shafts

ABSTRACT

A cam is capable of angular movement on a cam shaft. A disc eccentrically mounted on the shaft is driven from the shaft via a pin connection and drives the cam by a similar connection. The degree of eccentricity of the disc, which is adjustable, determines the acceleration and deceleration given to the cam as it rotates. There is disclosed an electro-mechanical servo system, responsive to the speed of an engine incorporating the cam and cam shaft, for controlling the eccentricity of the disc in dependence on the speed. Also disclosed is an electronic servo system for the same purpose.

[ Mar. 12, 1974 1 DEVICES FOR MOVING CAMS RELATIVE TO THEIR DRIVING SHAFT S [75] inventors: Lodovico Raggi, Milan, Italy; David A. Parker, Rugby; John Noddings, Coventry, both of England [73] Assignee: Associated Engineering Limited, Leamington Spa, Warwickshire, England [22] Filed: Dec. 4, 1972 [21] Appl. No.: 311,473

PULSE GEN.

2,829,327 4/1958 Cluwen 318/325 3,314,408 4/1967 Fenton 123/90.l7 3,516,394 6/1970 Nichols.... 123/9017 3,633,555 l/l972 Raggi 123/90.l7 3,638,624 2/1972 O'Grady 123/9018 Primary Examiner-B. Dobeck Attorney, Agent, or Firm-Mason, Mason & Albright 5 7] ABSTRACT A cam is capable of angular movement on a cam shaft. A disc eccentrically mounted on the shaft is driven from the shaft via a pin connection and drives the cam by a similar connection. The degree of eccentricity of the disc, which is adjustable, determines the acceleration and deceleration given to the cam as it rotates. There is disclosed an electro-rnechanical servo system, responsive to the speed of an engine incorporating the cam and cam shaft, for controlling the eccentricity of the disc in dependence on the speed. Also disclosed is an electronic servo system for the same purpose.

16 Claims, 5 Drawing Figures [30] Foreign Application Priority Data Dec. 2, 1971 Great Britain 55,902/71 [52] US. Cl 318/599, 318/666, 318/653, 123/9018, 290/40 [51] Int. Cl G05b 11/28, F011 1/34 [58] Field of Search 318/325, 666, 653, 599; 123/90.17, 90.18; 290/40, 41

[56] References Cited UNITED STATES PATENTS 2,550,603 4/1951 Russell 318/325 X FREQ/VOLT CON VERTOR COMPARATOR PULSE SHAPER 76 PATENTED 3.796.939

SHEET 1 UF 3 FREQ/VOLT v+ 65 CON VER TOR PULSE I #53 69 74 67 6250 33 COMP/IRA 70R 6 PULSE 77 MOTOR SHAPER 75 PULSE GEN. 71 72 PATENTEDHAR 12 I974 SHEET 2 OF 3 Fla PATENTEDMARIZ mm 31796339 sum 3 [1F 3 mm M W W W WW NE Aw i vi C DE DEVICES FOR MOVING CAMS RELATIVE TO THEIR DRIVING SHAFIS The invention relates to arrangements for angularly moving a cam relative to its driving shaft and is applicable particularly, but not exclusively, to an internal combustion engine to vary the movement of the cams which control the inlet and/or exhaust valves of the engine.

According to the invention, there is provided apparatus for adjusting the angular position of a cam on a shaft, comprising a cam driving member movable eccentrically of and connected for rotation by the shaft for rotating the cam and giving it a controlled advancement and retardation relative to the shaft in dependence on the degree of eccentricity of the said member, and servo means for determining the eccentricity of the said member and incorporating a feedback loop.

According to the invention, there is also provided apparatus for varying the angular position of a cam with respect to a camshaft in an internal combustion engine in which the cam is capable of angular movement relative to the camshaft, comprising a cam driving member movable eccentrically of and connected for rotation by the camshaft for rotating the cam with the shaft and giving the cam periodic acceleration and deceleration relative to the camshaft in dependence on the eccentricity of the cam driving member a rotatable member driven by the engine and carrying a mass which is movable in dependence on centrifugal force generated by rotation of the rotatable member, a first control member connected to be positioned by the movable mass in dependence on the speed of the engine, an electrical driving motor connected to determine the eccentricity of the said cam driving member, a second control member juxtaposed with the first control member and connected to have is instantaneous position determined by the instantaneous eccentricity of the cam driving member, and electrical switch means connected to detect a change in the relative position of the two control members from a predetermined datum relationship whereby to control the energisation of the electrical driving motor so that it alters the eccentricity of the cam driving member in a sense such as to reestablish the datum relationship of the two control members.

According to the invention, there is further provided apparatus for adjusting the angular position of a cam with respect to a camshaft in an internal combustion engine in which the cam is capable of angular movement relative to the camshaft, comprising a cam driving member movable eccentrically of and connected for rotation by the shaft for rotating the cam with the shaft and giving the cam a controlled pattern of acceleration and deceleration relative to the shaft in dependence on the degree of eccentricity of the cam driving member, input means responsive to the speed of the engine to produce an electrical input signal dependent on the said speed, means responsive to the degree of eccentricity of the cam driving member to produce an electrical feedback signal dependent thereon, comparing means for comparing the input and feedback signals to produce an error signal dependent on the difference (if any) between the two signals compared, and an electrical driving motor connected to be energised in response to the error signal so as to adjust the eccentricity of the cam driving member in a direction tending to reduce the difference between the input and feedback signals to zero.

Two arrangements embodying the invention and for varying the angular position of a cam on its shaft will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 shows diagrammatically one of the arrangements;

FIG. 2 is a schematic diagram of the other of the arrangements;

FIG. 3 is an electrical circuit diagram of part of the arrangement of FIG. 1;

FIG. 4 is a detailed circuit diagram of part of the arrangement of FIG. 2; and

FIG. 5 is a circuit diagram showing a modification to part of the circuit of FIG. 4.

In FIG. 1, a camshaft 5 is driven by any suitable means. A cam 15 is rotatably mounted on the camshaft 5, but otherwise axially located, and on being rotated moves a tappet 16.

A collar 8 is rigidly attached to the camshaft 5 and has a projecting pin 8a engaged in a radial groove 10 formed in a disc or intermediate member 9. On the other side of the disc 9 is a second radial groove 11 which is symmetrical to and preferably at an angle of to the first groove 10. A pin 15a projects from the side of the cam 15 to engage in the groove 1 1. The disc 9 has a central hole 12 larger than the camshaft 5 so that the disc 9 can be moved radially.

An arm 13 carries the disc 9 in a rotatable bearing 14, the arm 13 being movable radially and angularly relative to the camshaft 5 by means of an eccentric 31 rotatably mounted in a bearing 30 of the arm 13. The eccentric 31 is driven, such as by a pin 32, from a control shaft 33 which is rotatable about its axis, by an electric motor 34.

The mechanism whereby radial movement of the arm 13 produces variable rotation of the cam 15 relative to the cam-sahft 5 and collar 8 is disclosed in detail in a co-pending U.S. Pat. application Ser. No. 46,208 now a U.S. Pat. No. 3,633,555 to which reference is made. Briefly, however, the cam 15 is rotated by virtue of its being coupled to the collar 8 via the disc 9 and the pins 8a and 15a. When the disc 9 is symmetrically positioned relative to the shaft 5, the cam 15 rotates at a constant speed equal to the speed of the shaft 5. When the disc 9 is moved to an eccentric position relative to the shaft (by movement of the arm 13), however, the cam no longer rotates at a constant speed but accelerates and decelerates during each revolution. This acceleration and deceleration is dependent on the position of the arm 13 and can be used to alter the engine valve timing advantageous-1y.

A speed-responsive device comprises a disc 35 mounted on the end of the camshaft 5 to which a pair of fly weights 36 are pivoted on pins 37 so as to be moved outwards by centrifugal force on rotation of the camshaft 5. The fly weights 36 have fingers 38 bearing against an axially movable member 39 which is biassed towards the camshaft 5 by a compression spring 40 reacting against stationary structure 41. As the speed of rotation of the camshaft 5 increases, the fly weights 36 will move outwards, causing the axially movable member 39 to move towards the right in FIG. 1.

In a slot 42, formed in the axially movable member 39, there is engaged a pin 43 fastened in a radially movable member 44. A portion of toothed rack 45 is carried by pins 46 in guides 47 formed in the radially movable member 44. The rack 45 has a limited travel relative to the radially movable member 44 in which it is centred by springs 48. A,toothed gear 49 meshing with the rack 45 is mounted on the control shaft 33, or its extension on the opposite side of the electric motor 34. Micro-switches 50, 51 carried by the radially movable member 44 act against opposite ends of the rack 45 and are arranged to make and break on small movements of the latter relative to the radially movable member 44.

FIG. 3 shows the electrical connections of the microswitches 50, SI and the electric motor 34 which is of a reversible type. A damping resistor 52 is connected into the circuit as shown in FIG. 3 to bring the electric motor 34 to rest rapidly when it is not energised.

When the axially movable member 39 is moved towards the right in FIG. 1 on increase of speed of the camshaft 5, the profile of the slot 42 causes the radially movable member 44 to be moved upwards in FIG. 1 by an amount which is predetermined and depends on the chosen shape of the slot 42. This movement causes the micro-switch 51 to be closed, which energizes the electric motor 54 to rotate the control shaft 33 and gear 49. Rotation of the control shaft 33 produces the required movement of the arm 13, whilst rotation of the gear 49 moves the rack 45 upwardly until the micro-switch 51 breaks the supply of electric current to the motor 34, when rotation of the control shaft 33 is stopped. The rack 34 is now in a position corresponding to the position of the yoke 44 as determined by the engine speed. Therefore, shaft 33 will have a positioned arm 13 in accordance with the engine speed. Similarly, reduction of the speed of the camshaft produces movement of the axially movable member 39 towards the left which in turn produces downward movement of the radially movable member 44 causing the micro-switch 50 to make, and rotate the control shaft 33 and gear 49 in opposite directions until the micro-switch 50 again breaks. Again, therefore, the arm 13 has been positioned in accordance with engine speed.

The relationship between the speed of the camshaft 5 and movement of the arm 13 can be determined as required by the profile of the slot 42. The speedresponsive device need not be driven directly by the camshaft 5, but could be driven at any other speed proportional thereto, such as by the crankshaft of an internal combustion engine.

In an alternative embodiment disclosed in the abovementioned co-pending application, the disc 9 is carried in an eccentric sleeve mounted in a fixed housing. In this design the eccentric positioning of the disc 9 relative to the camshaft 5 is effected by rotation of the eccentric sleeve. This sleeve may be rotated, through suitable gearing, by shaft 33 of FIG. 1 herein.

FIGS. 2, 4 and 5 show an alternative embodiment to FIG. 1 in which the adjustment of movement of the cam relative to the camshaft 5 in dependence on camshaft speed is achieved electronically.

In FIG. 2, the control shaft 33 is rotated in one direction to adjust the movement of the cam 15 in one sense and in the opposite direction to adjust the cam oppositely, as in FIG. 1. Rotation of the control shaft 33 is produced by the electric motor 34 of which the armature is fed through a commutator 60 supplied through a constant current device 61. The motor 34 has a split field winding 62 of which the centre tapping 63 is connected to the positive power feed line.

A toothed wheel 64 is driven by a shaft 65 at a speed proportional to the speed of rotation of the camshaft 5. In an internal combustion piston engine, the toothed wheel 64 may conveniently be the engine flywheel having starter motor gear teeth, or a gear or chain sprocket in the camshaft drive from the engine crankshaft. The toothed wheel 64 is of ferromagnetic material and an inductive transducer 66 of suitable type is positioned close to the toothed wheel 64 so that rotation of the latter produces an electrical pulse froom the transducer 66 as each tooth passes. If preferred, any other suitable transducer may be used, such as a capacitive or electrooptical type, provided that it emits a stream of electrical pulses of which the frequency is proportional to the speed of rotation of the camshaft 5.

The stream of electrical pulses is fed to a pulse shaper and amplifier 67 which emits a stream of corresponding pulses each of constant height and width. These pulses pass to a frequency-to-voltage converter 68 of which the output voltage in the line 69 is proportional to the speed of rotation of the camshaft 5 or is in some predetermined relationship to it. The speed-dependant voltage in the line 69 is fed to a comparator 70.

The control shaft 33 is connected to move the slider 71 of a potentiometer 72 connected across the power supply lines. In this way a voltage will occur on the slider 71 which is proportional, or directly related to, the angular position of the control shaft 33. This control shaft voltage is fed through a line 73 to the comparator 70.

When the control shaft voltage is less than the speeddependant voltage, the comparator emits a voltage in theline 74 proportional to the difference between the control shaft and speed dependant voltages. The line 74 feeds an increase pulse generator 75 which emits a stream of pulses of constant height and width, of which the frequency rises from a minimum value in proportion to the voltage in the line 74. This stream of pulses is fed to one half of the field winding 62, causing the electric motor 34 to tend to inch the control shaft 33 round in a direction which will increase the movement of the cam 16 relative to the camshaft 5.

When the control shaft voltage is greater than the speed dependant voltage the comparator 70 emits a voltage along a line 76 of which the voltage is again proportional to the difference between the control shaft and speed dependant voltages. Line 76 feeds a decrease pulse generator 77 which functions in the same manner as the increase pulse generator 75, but is connected to the other half of the field winding 62. When there is no difference between the control shaft voltage and the speed dependant voltage, both the pulse generators 75 and 77 will emit pulses of identical height, width, frequency, and therefore mean current so that the electric motor 34 will not rotate. When the pulse frequency from one of the pulse generators 75 or 77 increases, its mean output current to the electric motor 34 thereby increases, causing the motor 34 to rotate at a speed proportional to the difference between the control shaft voltage and speed dependant voltage.

FIG. 4 shows in more detail the system shown in FIG. 2.

Pulses from the transducer 66 are amplified by a transducer amplifier which includes a transistor T1 and associated resistances R1, R2 and condenser Cll. Diodes DI and D2 remove voltage pulses which could damage the transistor. The pulses from the transducer amplifier are further shaped and limited by a capacitor C2, resistance R3, zener diode D3 and diode D4, from which they pass to a monostable circuit 82, of known kind, which includes transistors T2, T3, capacitors C3, C4 and resistances R4, R5, R6. For each input pulse the monostable circuit 81 emits a corresponding pulse of fixed height and width, the latter pulses being amplified by a driver circuit 83 comprising transistor T4 and resistors R7, R8, R9. The stream of pulses, of which the frequency is proportional to camshaft speed, is smoothed by capacitors C5, C6 to produce the aforementioned speed dependant voltage.

If it is required that the relationship between movement of cam position and speed should be non-linear, the smoothed voltage produced by the capacitors C5, C6 may be modified by using one or more circuit elements 84 of the type shown in FIG. 5. These consist of resistances R10, R11, R12 and R13 in combination with diodes D5 and D6 as shown in FIG. 5. By fusing elements of this type the relationship between the speed dependant voltage and camshaft speed may be altered as required.

The part of FIG. 4 so far described is fed with a suitable voltage provided by a resistor R14, zener diode D7 and capacitor C7. The potentiometer 72 is also connected across this stabilised voltage, as shown.

The control shaft voltage in the line 73 and the speed dependant voltage in the line 69 are fed to the comparator 70 which includes transistors T5, T6, T7, zener diode D8 and resistors R15, R16, R17, R18. The output lines 74, 76 of the comparator 70 are connected respectively to the increase pulse generator 75 and the decrease pulse generator 77. The increase pulse generator 75 includes transistors T7, T8, T9, diodes D9, D10, capacitors C8, C9, resistors R19, R20, R21, R22, R23 and an output amplifier transistor T10. As the voltage in the input line 74 arises from its quiescent value the increase pulse generator 75 will emit a stream of pulses along its output line 85, the pulses being of constant height and width but of frequency which increases from its minimum value with input voltage. Line 85 feeds these pulses to the upper half of the field winding 62. Each pulse will thereby tend to step" the motor 34.

Similarly, the decrease pulse generator 77 includes transistors T11, T12, T13, diodes D11, D12, capacitors C10, C11, resistors R24, R25, R26, R27, R28 and an output amplifier transistor T14. The decrease pulse generator 77 operates in the same manner as the increase pulse generator 76 described above except that its output line 86 is connected to the lower half of the field winding 62. In this way output pulses in the line 86 tend to rotate the motor 34 in the opposite direction to the pulses in the line 85. Damping of the motor movement is provided by diodes D13, D14.

In a modification, the transducer 66 and the associated transistor T1 can be dispensed with, and speed dependant pulses can be fed directly to capacitor C via a resistor connected to the low tension side of the contact breaker (not shown) of the usual ignition system of an internal combustion spark ignition engine.

Instead of the comparator 70 being fed with a speed dependant signal along the line 69, it could be fed with a signal of which the voltage changes in a predetermined manner in dependance on an engine parameter other than speed. For example the parameter could be a function of engine torque as related to the opening position of an inlet throttle valve or the pressure in an inlet duct or manifold. Either the throttle valve or a spring-biased bellows or diaphragm operated on by inlet pressure could move the slider of a potentiometer to generate the torque-dependant voltage in line 69. By a suitable resistance network the torque-dependant voltage could be combined with the speed dependant voltage of the toothed wheel 64, transducer 66 and circuits 67 and 68 are retained.

What is claimed is:

1. Apparatus for adjusting the angular position of a cam on a shaft, wherein the cam is capable of angular movement relative to the shaft, comprising a cam driving member,

means mounting the cam driving member on the shaft and constraining it for limited movement eccentrically of the shaft,

means connecting the cam driving member to the shaft for rotation thereby,

means connecting the cam driving member to the cam so as to rotate the cam and give it a controlled advancement and retardation relative to the shaft in dependence on the degree of eccentricity of the said member, and

servo means connected to control the eccentricity of the cam driving member in dependence on a signal indicative of a desired eccentricity and incorporating a feedback loop.

2. Apparatus according to claim 1, in which the said servo means comprises input means responsive to the value of a parameter on which the cam movement is to depend to produce an input signal dependent on that value,

feedback means connected to respond to the actual eccentricity of the cam driving member to produce a feedback signal dependent thereon,

comparing means connected to compare the input and feedback signals whereby to produce an error signal dependent on the difference (if any) between them, and

an electrical drive motor responsive to the error signal and drivingly connected to the cam driving member to alter its said eccentricity in such a direction as to tend to balance the input and feedback signals.

3. Apparatus according to claim 2, in which the input means comprises means responsive to the value of said parameter to produce an electrical pulse train whose pulse repetition frequency is dependent on the said value, and means connected to receive the said pulse train to produce as the said input signal an electrical signal dependent on the mean value of the pulse train,

the said feedback means comprises means connected to sense the instantaneous eccentricity of the movable member to produce an electrical feedback signal in dependence thereon, and

the said comparing means is connected to receive the input and feedback electrical signals whereby to produce the said electrical error signal in dependence on the sign of the difference (if any) between the two signals compared.

4. Apparatus according to claim 3, in which the comparing means comprises a first pulse circuit connected to produce a first pulse train whose frequency increases when the difference between the input and feedback signals changes in one direction and decreases when the difference changes in the opposite direction, and a second pulse circuit connected to produce a second pulse train whose frequency increases when the difference changes in the said other direction and decreases when the said difference changes in the said one direction, and

the said electrical drive motor is connected to receive the first and second pulse trains and to be driven in response to the algebraic mean value of them.

5. Apparatus according to claim 4, in which the electrical drive motor has a split field winding one end of which is connected to receive the first pulse train and the other end of which is connected to receive the second pulse train.

6. Apparatus according to claim 4, in which the cam is a cam in an internal combustion spark ignition engine, and in which the input means comprises means for picking up electrical pulses produced in the ignition system of the engine.

7. Apparatus according to claim 2, in which the input means comprises a first movably mounted control member mechanically responsive to the value of the said parameter and connectedto move in a direction and for a distance dependent, respectively, on the sign and magnitude of any change in the said value, whereby the said input signal is the position of the first control member,

the said feedback means comprises a second control member mounted for movement adjacent and relative to the first control member and connected to have its position determined by the eccentricity of the cam driving member, whereby its position constitutes the feedback signal, and

the said comparing means includes electric detection means responsive to a change, from a predetermined datum relationship, in the relative positions of the first and second control members to produce an electrical error signal dependent on the direction of the relative movement.

8. Apparatus according to claim 7, in which the electrical detection means comprises first electrical switch means mounted on one of the control members so as to be actuated by the other thereof when the two control members move relatively in one direction from the datum relationship, second electrical switch means mounted on one of the control members to be actuated by the other thereof when the two control members move relatively in the opposite direction, and

means electrically connecting the electrical driving motor for energisation through the two switch means whereby the motoris driven in one direction when one switch means is actuated and in the other direction when the other switch means is actuated.

9. Apparatus according to claim 8, invwhich the second control member comprises a member mounted for linear movement, and including rack and pinion gearing driven by the said driving motor and connected to move the second control member linearly.

10. Apparatus according to claim 9, including means locating the first control member on the second control member but allowing limited relative movement between the two members in the said directions, and means resiliently biasing the two members into the said datum relationship.

11. Apparatus according to claim 7, in which the said parameter'is the rotational speed of a driven shaft, and including a mass mounted on the driven shaft for rotation therewith and capable of movement radially in response to centrifugal force, and including means responsive to the radial movement of the mass for positioning the first control member.

12. Apparatus according to claim 11, including an intermediate member mounted for movement in response to the radial movement of the mass, and a pin and slot connection interconnecting the intermediate member and first control member for moving the first control member correspondingly.

13. Apparatus according to claim 7, in which the cam is a cam of an internal combustion engine, and the said parameter is the rotational speed of the engine.

14. Apparatus for varying the angular position of a cam with respect to a cam shaft in an internal combustion engine wherein the cam is capable of angular movement relative to the cam shaft, comprising a cam driving member,

means mounting the cam driving member on the cam shaft for limited movement eccentrically thereof,

first drive means drivingly connecting the cam driving member for rotation with the cam shaft,

second drive means drivingly connecting the cam driving member to the cam so as to rotate the cam with the shaft and to give the shaft periodic acceleration and deceleration relative to the cam shaft in dependence upon the eccentricity of the cam driving member,

a rotatable member driven by the engine,

a mass mounted on the rotatable member for rotation therewith and for movement radially thereof in dependence on the centrifugal force generated by the rotation,

a first control member mounted for limited movement in two opposite directions and connected to be positioned in response to the radial movement of the said mass in dependence on the speed of the engine,

an electrical driving motor drivingly connected to the cam driving member to determine its eccentricity,

a second control member mounted for movement adjacent and relative to the first control member and connected to have its instantaneous position determined by the instantaneous eccentricity of the cam driving member,

electrical switch means mounted on one of the control members for actuation by the other thereof to detect a change in the relative position of the two control members from a predetermined datum relationship, and

an energisation circuit for the driving motor connected to be supplied through the said switch means whereby to control the energisation of the driving motor so that it alters the eccentricity of the cam driving member in such a sense as to reestablish the datum relationship of the two control members.

15. Apparatus for adjusting the angular position of a cam with respect to a cam shaft in an internal combustion engine wherein the cam is capable of angular movement relative to the cam shaft, comprising a cam driving member,

means mounting the cam driving member on the cam shaft for limited movement eccentrically thereof,

first drive means drivingly connecting the cam driving member for rotation with the cam shaft,

second drive means drivingly connecting the cam driving member to the cam for rotating the cam with the cam shaft and giving the cam periodic acceleration and deceleration relative to the shaft in dependence upon the eccentricity of the cam driving member,

input means connected to respond to the speed of the engine to produce an electrical input signal dependent on the speed,

a transducer responsive to the degree of eccentricity of the cam driving member to produce an electrical feedback signal dependent thereon,

comparing means connected to receive the input and feedback signals to produce an error signal dependent on the difference (if any) between the two signals compared, and

an electrical driving motor connected to be energised in response to the error signal and drivingly connected to the cam driving member to adjust the eccentricity thereof in a direction tending to reduce the difference between the input and feedback signals to zero.

16. Apparatus according to claim 15, in which the input means comprises means to produce a train of electrical pulses whose pulse repetition frequency is dependent on the speed of rotation of the engine, and means responsive to the pulses to produce as the said input signal an electrical signal dependent on the mean value of the pulses in the pulse train,

the comparing means includes first and second electrical pulse circuits connected to produce respective pulse trains of equal pulse height and width, the pulse repetition frequency of one pulse train increasing when the difference between the input and feedback signals changes in one direction and decreasing when it changes in the other direction, and the pulse repetition frequency of .the other pulse train increasing when the said difference changes in the said other direction and decreasing when the said difference changes in the said one direction, and i the said energisation circuit includes means for energising the electric motor in dependence on the difference between the mean values of the two pulse trains. 

1. Apparatus for adjusting the angular position of a cam on a shaft, wherein the cam is capable of angular movement relative to the shaft, comprising a cam driving member, means mounting the cam driving member on the shaft and constraining it for limited movement eccentrically of the shaft, means connecting the cam driving member to the shaft for rotation thereby, means connecting the cam driving member to the cam so as to rotate the cam and give it a controlled advancement and retardation relative to the shaft in dependence on the degree of eccentricity of the said member, and servo means connected to control the eccentricity of the cam driving member in dependence on a signal indicative of a desired eccentricity and incorporaTing a feedback loop.
 2. Apparatus according to claim 1, in which the said servo means comprises input means responsive to the value of a parameter on which the cam movement is to depend to produce an input signal dependent on that value, feedback means connected to respond to the actual eccentricity of the cam driving member to produce a feedback signal dependent thereon, comparing means connected to compare the input and feedback signals whereby to produce an error signal dependent on the difference (if any) between them, and an electrical drive motor responsive to the error signal and drivingly connected to the cam driving member to alter its said eccentricity in such a direction as to tend to balance the input and feedback signals.
 3. Apparatus according to claim 2, in which the input means comprises means responsive to the value of said parameter to produce an electrical pulse train whose pulse repetition frequency is dependent on the said value, and means connected to receive the said pulse train to produce as the said input signal an electrical signal dependent on the mean value of the pulse train, the said feedback means comprises means connected to sense the instantaneous eccentricity of the movable member to produce an electrical feedback signal in dependence thereon, and the said comparing means is connected to receive the input and feedback electrical signals whereby to produce the said electrical error signal in dependence on the sign of the difference (if any) between the two signals compared.
 4. Apparatus according to claim 3, in which the comparing means comprises a first pulse circuit connected to produce a first pulse train whose frequency increases when the difference between the input and feedback signals changes in one direction and decreases when the difference changes in the opposite direction, and a second pulse circuit connected to produce a second pulse train whose frequency increases when the difference changes in the said other direction and decreases when the said difference changes in the said one direction, and the said electrical drive motor is connected to receive the first and second pulse trains and to be driven in response to the algebraic mean value of them.
 5. Apparatus according to claim 4, in which the electrical drive motor has a split field winding one end of which is connected to receive the first pulse train and the other end of which is connected to receive the second pulse train.
 6. Apparatus according to claim 4, in which the cam is a cam in an internal combustion spark ignition engine, and in which the input means comprises means for picking up electrical pulses produced in the ignition system of the engine.
 7. Apparatus according to claim 2, in which the input means comprises a first movably mounted control member mechanically responsive to the value of the said parameter and connected to move in a direction and for a distance dependent, respectively, on the sign and magnitude of any change in the said value, whereby the said input signal is the position of the first control member, the said feedback means comprises a second control member mounted for movement adjacent and relative to the first control member and connected to have its position determined by the eccentricity of the cam driving member, whereby its position constitutes the feedback signal, and the said comparing means includes electric detection means responsive to a change, from a predetermined datum relationship, in the relative positions of the first and second control members to produce an electrical error signal dependent on the direction of the relative movement.
 8. Apparatus according to claim 7, in which the electrical detection means comprises first electrical switch means mounted on one of the control members so as to be actuated by the other thereof when the two control members move relatively in one direction from the datum relationship, second electrical switch means mounted on one of the control members to be actuated by the other thereof when the two control members move relatively in the opposite direction, and means electrically connecting the electrical driving motor for energisation through the two switch means whereby the motor is driven in one direction when one switch means is actuated and in the other direction when the other switch means is actuated.
 9. Apparatus according to claim 8, in which the second control member comprises a member mounted for linear movement, and including rack and pinion gearing driven by the said driving motor and connected to move the second control member linearly.
 10. Apparatus according to claim 9, including means locating the first control member on the second control member but allowing limited relative movement between the two members in the said directions, and means resiliently biasing the two members into the said datum relationship.
 11. Apparatus according to claim 7, in which the said parameter is the rotational speed of a driven shaft, and including a mass mounted on the driven shaft for rotation therewith and capable of movement radially in response to centrifugal force, and including means responsive to the radial movement of the mass for positioning the first control member.
 12. Apparatus according to claim 11, including an intermediate member mounted for movement in response to the radial movement of the mass, and a pin and slot connection interconnecting the intermediate member and first control member for moving the first control member correspondingly.
 13. Apparatus according to claim 7, in which the cam is a cam of an internal combustion engine, and the said parameter is the rotational speed of the engine.
 14. Apparatus for varying the angular position of a cam with respect to a cam shaft in an internal combustion engine wherein the cam is capable of angular movement relative to the cam shaft, comprising a cam driving member, means mounting the cam driving member on the cam shaft for limited movement eccentrically thereof, first drive means drivingly connecting the cam driving member for rotation with the cam shaft, second drive means drivingly connecting the cam driving member to the cam so as to rotate the cam with the shaft and to give the shaft periodic acceleration and deceleration relative to the cam shaft in dependence upon the eccentricity of the cam driving member, a rotatable member driven by the engine, a mass mounted on the rotatable member for rotation therewith and for movement radially thereof in dependence on the centrifugal force generated by the rotation, a first control member mounted for limited movement in two opposite directions and connected to be positioned in response to the radial movement of the said mass in dependence on the speed of the engine, an electrical driving motor drivingly connected to the cam driving member to determine its eccentricity, a second control member mounted for movement adjacent and relative to the first control member and connected to have its instantaneous position determined by the instantaneous eccentricity of the cam driving member, electrical switch means mounted on one of the control members for actuation by the other thereof to detect a change in the relative position of the two control members from a predetermined datum relationship, and an energisation circuit for the driving motor connected to be supplied through the said switch means whereby to control the energisation of the driving motor so that it alters the eccentricity of the cam driving member in such a sense as to re-establish the datum relationship of the two control members.
 15. Apparatus for adjusting the angular position of a cam with respect to a cam shaft in an internal combustion engine wherein the cam is capable of angular movement relative to the cam shaft, comprising a cam driving member, means mounting the cam driving member on the cam shaft for limited movement eccentrically thereof, first drive means drivingly connecting the cam driving member for rotation with the cam shaft, second drive means drivingly connecting the cam driving member to the cam for rotating the cam with the cam shaft and giving the cam periodic acceleration and deceleration relative to the shaft in dependence upon the eccentricity of the cam driving member, input means connected to respond to the speed of the engine to produce an electrical input signal dependent on the speed, a transducer responsive to the degree of eccentricity of the cam driving member to produce an electrical feedback signal dependent thereon, comparing means connected to receive the input and feedback signals to produce an error signal dependent on the difference (if any) between the two signals compared, and an electrical driving motor connected to be energised in response to the error signal and drivingly connected to the cam driving member to adjust the eccentricity thereof in a direction tending to reduce the difference between the input and feedback signals to zero.
 16. Apparatus according to claim 15, in which the input means comprises means to produce a train of electrical pulses whose pulse repetition frequency is dependent on the speed of rotation of the engine, and means responsive to the pulses to produce as the said input signal an electrical signal dependent on the mean value of the pulses in the pulse train, the comparing means includes first and second electrical pulse circuits connected to produce respective pulse trains of equal pulse height and width, the pulse repetition frequency of one pulse train increasing when the difference between the input and feedback signals changes in one direction and decreasing when it changes in the other direction, and the pulse repetition frequency of the other pulse train increasing when the said difference changes in the said other direction and decreasing when the said difference changes in the said one direction, and the said energisation circuit includes means for energising the electric motor in dependence on the difference between the mean values of the two pulse trains. 